1. Field of the Invention
The present invention relates to a surface acoustic wave substrate having a LiNbO3 substrate propagating piezoelectric leaky surface wave along an X-axis direction on a cut plane of a rotated Y plate, and a thin film of SiO2 or the like formed on the cut plane. More particularly, the invention relates to a surface acoustic wave substrate and a surface acoustic wave functional element having superior temperature characteristics.
2. Description of the Related Art
A surface acoustic wave functional element provided with an interdigital electrode on a surface of a piezoelectric substrate, has been widely used for a filter of an intermediate frequency band of a television set, a filter of a mobile communication equipment and so forth. The surface acoustic wave functional element has an electrode for exciting a surface acoustic wave and an electrode for receiving the surface acoustic wave on a surface of a substrate having piezoelectric function.
As a piezoelectric substrate to be used for the surface acoustic wave functional element in the prior art, a material having large electromechanical coupling coefficient k2 has been used. However, the surface acoustic wave functional element using the substrate made of material having large electromechanical coupling coefficient k2 generally has bad temperature characteristics and thus encounters a problem of lacking of temperature stability.
On the other hand, the surface acoustic wave functional element employing a monocrystalline piezoelectric substrate, such as ST-cut quartz, LST-cut quartz and so forth, demonstrates superior temperature stability, but has small electromechanical coupling coefficient k2. Therefore, when it is used as the filter, insertion loss becomes large. Also, such surface acoustic wave functional element cannot be used for a filter having wide band width.
Therefore, there have been devised a SiO2/LiNbO3 substrate and a SiO2/LiTaO3 substrate, which are fabricated by using a LiNbO3 substrate and a LiTaO3 substrate as the substrates having superior temperature stability and large electromechanical coupling coefficient k2, and depositing thereon SiO2 films having small linear expansion coefficient and opposite temperature characteristics, respectively. Effectiveness of such SiO2/LiNbO3 substrate and SiO2/LiTaO3 substrate has been discussed in Yamanouchi, Iwahashi and Shibayama, xe2x80x9cTemperature Dependence of Rayleigh Waves and Piezoelectric Leaky Surface Waves in Rotated Y-Cut LiTaO3 and SiO2/LiTaO3 Structuresxe2x80x9d Wave Electronics, Vol. 3, December, 1979, or Yamanouchi and Hayama, xe2x80x9cSAW Properties of SiO2/128xc2x0 Y-X LiNbO3 Structure Fabricated by Magnetron Sputtering Techniquexe2x80x9d, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-31, No. 1, January, 1984. These substrates are recommended to apply as high stability oscillator and filters using normally bi-directional interdigital electrode.
However, substrate having further greater electromechanical coupling coefficient k2 than the conventional substrate and having superior temperature stability has been required.
The present invention has been worked out in view of the shortcomings in the prior art. Therefore, it is an object of the present invention to provide a surface acoustic wave substrate and a surf ace acoustic wave functional element having an electromechanical coupling coefficient k 2 greater than the prior art and good temperature characteristics.
According to a first aspect of the present invention, there is provided a surf ace acoustic wave substrate comprising:
a piezoelectric or electrostrictive substrate having large electromechanical coupling coefficient; and
a thin film formed on the substrate and having variation characteristics of frequency of a surface acoustic wave relative temperature variation opposite to that of the substrate,
wherein the substrate is a LiNbO3 substrate having a cut angle of rotated Y plate within a range greater than or equal to xe2x88x9210xc2x0 and smaller than or equal to +30xc2x0 and propagating a piezoelectric leaky surface wave having a propagation velocity higher than that of a Rayleigh type surface acoustic wave along X-axis direction or within a range of xc2x15xc2x0 with respect to X-axis direction, and
a value of H/xcex falls within a range from 0.05 to 0.35, where H is the film thickness of the thin film, and xcex is the wavelength of operating center frequency of the piezoelectric leaky surface wave.
It is preferred that the cut angle of rotated Y plate of the substrate is in a range greater than or equal to 0xc2x0 and smaller than or equal to +20xc2x0, and the value of H/xcex falls within a range from 0.1 to 0.35. In the alternative, it is preferred that the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +20xc2x0 and smaller than or equal to +30xc2x0, and the value of H/xcex falls within a range from 0.15 to 0.35. By selecting the cut angle of rotated Y plate and H/xcex within the foregoing ranges, the temperature coefficient of frequency (TCF) as measured at 25xc2x0 C. can be zero or quite small.
In the surface acoustic wave substrate of the invention, preferably, the temperature coefficient of frequency (TCF) as measured at 25xc2x0 C. is in a range from xe2x88x9230 ppm/xc2x0 C. to +30 ppm/xc2x0 C. In addition, it is preferred that the electromechanical coupling coefficient k2 of the piezoelectric leaky surface wave is greater than or equal to 0.155 and the electromechanical coupling coefficient kR2 of a Rayleigh wave component is smaller than or equal to 0.01.
The ranges of the temperature coefficient of frequency (TCF) and the electromechanical coupling coefficient k2 may be achieved by selecting the cut angle of rotated Y plate and H/xcex within a range set forth in any of the following (1) to (5):
(1) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to xe2x88x9210xc2x0 and smaller than or equal to xe2x88x925xc2x0, and the value of H/xcex falls within a range from 0.07 to 0.31;
(2) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to xe2x88x925xc2x0 and smaller than or equal to +10xc2x0, and the value of H/xcex falls within a range from 0.115 to 0.31;
(3) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +10xc2x0 and smaller than or equal to +15xc2x0, and the value of H/xcex falls within a range from 0.16 to 0.31;
(4) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +15xc2x0 and smaller than or equal to +20xc2x0, and the value of H/xcex falls within a range from 0.2 to 0.31; and
(5) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +20xc2x0 and smaller than or equal to +30xc2x0, and the value of H/xcex falls within a range from 0.25 to 0.31.
According to a second aspect of the present invention, there is provided a surface acoustic wave functional element comprising a surface acoustic wave substrate as set forth above, the element including:
an exciting or receiving region having an interdigital electrode for exciting or receiving the piezoelectric leaky surface wave formed at an interface between the surface of the substrate and the thin film; and
a propagating region having a structure for electrically shorting between the substrate and the thin film or a shorting type grating electrode structure formed at an interface between the surface of the substrate and the thin film.
According to a third aspect of the present invention, there is provided a surface acoustic wave functional element comprising a substrate including: a piezoelectric or electrostrictive substrate having large electromechanical coupling coefficient; and a thin film formed on the substrate and having variation characteristics of frequency of a surface acoustic wave relative temperature variation opposite to that of the substrate,
wherein the substrate is a LiNbO3 substrate having a cut angle of rotated Y plate within a range greater than or equal to xe2x88x9210xc2x0 and smaller than or equal to +30xc2x0 and propagating a piezoelectric leaky surface wave having a propagation velocity higher than that of a Rayleigh type surface acoustic wave along X-axis direction or within a range of xc2x15xc2x0 with respect to X-axis direction, and
a value of H/xcex falls within a range from 0 to 0.35 in an exciting or receiving region, and within a range from 0.05 to 0.35 in a propagating region, where H is the film thickness of the thin film, and xcex is the wavelength of operating center frequency of the surface acoustic wave.
Even in this case, the exciting or receiving region may have an interdigital electrode for exciting or receiving the piezoelectric leaky surface wave formed at an interface between the surface of the substrate and the thin film; and the propagating region may have a structure for electrically shorting between the substrate and the thin film or a shorting type grating electrode structure formed at an interface between the surface of the substrate and the thin film. With the structure set forth above, the electromechanical coupling coefficient k2 becomes large in the exciting or receiving region and the temperature coefficient of frequency (TCF) becomes small in the propagating region.
In addition, by setting H/xcex within a preferred range in the exciting or receiving region and in the propagating region, it becomes possible to obtain a surface acoustic wave functional element in which the electromechanical coupling coefficient k2 of the piezoelectric leaky surface wave is greater than or equal to 0.155 in the exciting or receiving region, and the temperature coefficient of frequency (TCF) as measured at 25xc2x0 C. is in a range from xe2x88x9230 ppm/xc2x0 C. to +30 ppm/xc2x0 C. in the propagating region.
In the surface acoustic wave functional element as set forth above, the interdigital electrode is preferably made of one metal selected from the group consisting of Al, Cu, Ti, W, Mo, Cr, Au and Ag or a combination or alloy of two or more metals thereof, and the propagating region is preferably provided with a conductive layer made of one metal selected from the group consisting of Al, Cu, Ti, W, Mo, Cr, Au and Ag or a combination or alloy of two or more metals thereof, as the structure for electrically shorting between the substrate and the thin film.
Accordingly, by employing any one of the surface acoustic wave functional elements as filter, wide band frequency characteristics and low insertion loss can be achieved.